When running in virtual environment a bootstrapping task will
install 'docker' python module in the environment. After that
prechecks and deploy phase will not find the module because
their 'ansible interpreter' is host wide. With this fix it is
advised to set an interpreter to the one in virtual environment

Closes-Bug: #1969812
Change-Id: I1bb6a3af3fa6346fff0a262e0c53d12c544f46db